My '92 YJ has the factory Dana 35C - Limited Slip with 3.07 gears (yuk!) and I love it, I've been offroading since I turned 16 and now I don't know how I ever survived without a posi.
As for an aftermarket carrier, and less stress on the axle I really don't know, but if you do any sort of offroading or if you get snow, I would recommend it.
I think "Jeep Chic" recently put a Powertrax in her rear end (no pun), maybe you can get more info from her.
"AN ARMED SOCIETY IS A POLITE SOCIETY"
"We, too, born to freedom, and believing in freedom, are willing to fight to maintain freedom. We and all others who believe as deeply as we do, would rather die on our feet than live on our knees." FDR
Blue Jeep Club Member #4
92 Canyon Blue Wrangler YJ 4.0L - with some fun goodies